Piezoelectric transformers

ABSTRACT

In a driving circuit for driving a piezoelectric transformer including a piezoelectric member provided with a pair of driving electrodes and an output electrode, there are provided a transformer having primary and secondary windings and a transistor. The collector electrode of the transistor is connected to one pole of a DC source through the primary winding, and the secondary winding and the driving electrodes are connected in series across the base and emitter electrodes thereby forming an oscillation circuit for producing a high frequency driving voltage.

I Unlted States Patent 11 1 1111 3,778,648 Kawada 1 Dec. 11, 1973 [541 PIEZOELECTRIC TRANSFORMERS 3,302,131 1/1967 Pyatt 331 116 R 1 1 111cm Takehiko Kawada, Yokohama, 312331333 25133? 2122112;51311111131111:3:3513131 51 Japan 3,054,967 9/1962 Gindi 331/116 R [73] Assignee: Denki Onkyo Company, Lt 3,435,368 3/1969 Fukuyo 331/116 R X y Japan 3,277,465 10/1966 Potter 310/116 R X 1 Filed! J 1972 Primary Examiner-J. D. Miller 2 A No: Assistant ExaminerMark I 1 pp Att0rney- C. Yardley Chittick et a1. [30] Foreign Application Priority Data [57] ABSTRACT July 5, 1971 Japan 46/49448 In a driving circuit for driving a piezoelectric trans 521 u.s.c1. ..310/s.1, 310/93, 331/116 R F l a? Piezoelectric member Pmvided [51] Int. Cl 1-10lv 7/00 wlth a of dnvmg electrodes and an output elec' [58] Field of Search 310/8.1; 331/116 R, the": are ransfmmer f I 3331/37 mary and secondary Wllldll'lgS and a trans1stor. The collector electrode of the Hammer 1s connected to [561 CM ZZZ 11222233551511;3:11;:21131131321325; UNITED STATES PATENTS are connected in series across the base and emitter 3,657,579 4/1972 Kramer 1. 310/8.1 lectrodes thereby forming an oscillation circuit for 3,679,918 7 1972 K6121 3l0/8.l producing a high frequency driving voltage. 3,689,781 9/1972 Kawada 310/8.l 3,382,447 5/1968 Racy 331/116 R 7 Claims, 4 Drawing Figures "=1 12 c 4] 1 l5 l4 1 PIEZOELECTRIC TRANSFORMERS BACKGROUND OF THE INVENTION This invention relates to a piezoelectric transformer and more particularly to an improved driving circuit thereof.

A piezoelectric transformer utilizes a ceramic bar of a ferroelectric material, such as lead titanate zirconate or the like. The ceramic bar is polarized and provided with a pair of opposed driving electrodes at one end and an output electrode at the other end. When applied with a driving voltage across the driving electrodes the piezoelectric barvibrates mechanically. As is well known in the art, when the frequency of the driving voltage is equal or nearly equal to the natural or resonance frequency of the piezoelectric bar, the efficiency is the highest. The vibrating frequency of the piezoelectric bar is determined mainly by the length thereof andis given by the following equation;

where V is a velocity of sound wave transmitting through the bar and L the length thereof. From this equation it can be noted that the vibrating frequency f is determined by the dimensions of the bar. In one example where the bar has dimensions of 56X 3.5 mm, the vibration frequency equals about 31 KHz at a )t/Z mode whereas about 62 KHz at a A mode and the frequency of the driving voltage is determined to be equal or nearly eaual to these vibration frequencies.

However, the resonance frequency of the piezoelectric transformer varies dependent upon the variations not only in the temperature but also in the impedance of the load connected to the output electrode.

For this reason, in order to operate the piezoelectric transformer at high efficiencies it is necessary to drive it with a frequency which varies in accordance with the variation in the resonance frequency of the piezoelectric transformer.

SUMMARY OF THE INVENTION It is an object of this invention to provide an improved driving circuit for a piezoelectric transformer capable of operating the piezoelectric transformer stably and at high efficiency irrespective of the variations in the ambient temperature and the load impedance.

Another object of this invention is to provide an improved driving circuit for a piezoelectric transformer which is simple in construction and yet can drive stably the piezoelectric transformer.

According to this invention there is provided a piezoelectric transformer comprising a piezoelectric member having a pair of driving electrodes at one end and an output electrode at the opposite end of the piezoelectric member, a transistor having a base electrode, a collector electrode and an emitter electrode, a transformer having a primary winding and a secondary winding, means to connect the collector electrode to one pole of a DC source through the primary winding and means to connect in series the secondary winding and the driving electrodes across the base and emitter electrodes of the transistor, thereby forming an oscillation circuit for producing a high frequency driving voltage.

BRIEF DESCRIPTION OF THE DRAWING The invention can be more fully understood from the DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 of the accompanying drawing shows the relationship between the driving frequency f and the absolute value of the input impedance I Z l across the driving electrodes and the phase difference P between the driving voltage and driving current. As shown, the absolute value of the imput impedance decreases to a minimum at the resonance frequency fr at which the phase difference P between the voltage and current becomes zero. On the otherhand the absolute value of the input impedance I Z l reaches a maximum value at the antiresonance frequency fe at which the phase difference P again reaches zero. For this reason it is possible to drive the piezoelectric transformer always at high efficiencies irrespective of the variations in the temperature and load impedance by controlling the driving frequency in such a manner that the phase difference between the current and voltage becomes zero and that the input impedance is maintained at a low value.

Referring now to FIG. 2 which shows a basic connection of a piezoelectric transformer, a piezoelectric bar 1 is provided with a pair of driving electrodes 11 and 12 applied on the opposite surfaces at one end of the bar and an output electrode 10 applied to the opposite end of the bar.

The'primary winding 31 of a transformer 3 is connected to the collector electrode 21 of a transistor 2 whereas the secondary winding 32 is connected in series with the driving electrodes across the base electrode 22 and the emitter electrode 23 of the transistor. The transistor and the transformer constitute an oscillator for generating a high frequency driving voltage. The connection is such that the phase of the collector output is reversed and the phase reversed current is fed back to the base electrode so that increase in the collectorcurrent results in the increase in the base-emitter current.

With the oscillation circuit constructed as above described, the phase of the output from the collector electrode 21 of transistor 2 is shifted by and the phase shifted output is then applied across the driving electrodes l1 and 12 of the piezoelectric bar 1. As above described, when the frequency of the driving voltage impressed across the driving electrodes 11 and 12 is the same as the resonance frequency of the piezoelectric bar, the input impedance becomes a minimum. In this manner, since there is always a difference of 180 between the voltages at the base electrode 22 and the collector electrode 21 the oscillator is assured to operate at a frequency at which the input impedance is the minimum and the feedback is the maximum or at the reso nance frequency of the piezoelectric bar.

For this reason, according to this invention, even when the resonance frequency of the piezoelectric transformer varies in response to the variation in the ambient temperature or the load impedance it is possible to drive the piezoelectric transformer always with a frequency equal to the resonance frequency thereof.

Although in the embodiment shown in FIG. 2, the transistor is of the emitter ground type, FIG. 3 shows a transistor of the base ground type. However, it will be clear that both embodiments operate in the same manner.

FIG. 4 shows a detailed connection diagram of one embodiment of the invention. The output electrode 10 of the piezoelectric transformer l'is connected to a load 15 via a suitable rectifier circuit shown as comprising a pair of diodes l3 and 14. The connection between transistor 2 and transformer 3 is identical to that shown in FIG. 2 except that a variable resistor 24 is connected between driving electrode 12 and the base electrode 22. A bias voltage is impressed upon the base electrode 22 from a juncture between resistors 25 and 26. Further, diodes 27 and 28 are provided for protecting the base and collector electrodes, respectively, of the transistor 2.

A capacitor 29 is connected between driving electrode 12 which is connected to the base electrode 22 of transistor 2 through variable resistor 24, and the emitter electrode 23 for the purpose of compensating for the phase lag of the signals of the base and collector electrodes which is caused by the charge storage time of the transistor, By a suitable adjustment of resistor 24, the capacitor 29 compensates for such phase lag.

The other driving electrode 11 is connected to earth potential E through the secondary winding 32 of transformer 3. In this embodiment, the driving circuit is energized from a source of commercial frequency 4 through a bridge rectifier circuit and a smoothing capacitor 41.

Since, one terminal of the source 4 is usually grounded, when grounding the load 15, it is necessary to pay due consideration, otherwise a short circuit will be resulted thus causing a damage to the source. More particularly, since the ground potential E and the earth potential of the chassis E are not equal, direct grounding of the chassis causes a large short circuit current to flow through the rectifier circuit 5. For this reason, according to this invention, the chassis is grounded through a capacitor 60 having a capacity of several picofarads. Thus, the capacitor 60 exhibits a high reactance for the commercial frequency but a low reactance for the output frequency (for example, 30 to 70 KHz) of the piezoelectric transformer. A resistor 61 having a value of several hundreds kilohms may be connected in parallel with the capacitor 60 to form a shunt circuit for the DC component of the load current.

From the foregoing description it will be clear that the invention provides an improved piezoelectric transformer provided with a novel driving circuit which is simple in construction, can drive the piezoelectric transformer at high efficiencies and simplifies the construction of the source rectifier. By using a voltage multiplying rectifier circuit on the output side of the piezoelectric transformer, it is possible to use it for charging devices and dust precipitating apparatus.

While the invention has been shown and described in terms of a preferred empodiment thereof, it should be understood that the invention is by no means limited to this particular embodiment and that many changes and modifications will be obvious to one skilled in the art without departing from the true spirit and scope of the invention. What is claimed is:

1. A piezoelectric transformer comprising a piezoelectric member having a pair of driving electrodes at one end and an output electrode at the opposite end of said piezoelectric member, a transistor having a base electrode, a collector electrode and an emitter electrode, a transformer having a primary winding and a secondary winding, means to connect said collector electrode to one pole of a DC source through said primary winding and means to connect in series said secondary winding and said driving electrodes across the base and emitter electrodes of said transistor, thereby forming an oscillation circuit for producing a high voltage driving voltage, and including a phase adjuster having a variable resistor connected between one of the driving electrodes and said base electrode and a capacitor connected between said one driving electrode and said emitter electrode.

2. The piezoelectric transformer according to claim 1 wherein the other pole of said DC source is grounded through a capacitor.

3. The piezoelectric transformer according to claim 2 wherein a shunt resistor is connected in parallel with said capacitor.

4. Apparatus according to claim 1 and including a diode connected between said collector and emitter electrodes and poled to protect said collector electrode.

5. Apparatus according to claim 4 and including a diode connected between said base and emitter electrodes and poled to protect said emitter electrode.

6. Apparatus according to claim 1 and including a resistor voltage divider connected across said DC source with an intermediate point on said voltage divider connected to said base electrode for biasing the collector base and emitter base junctions.

7. Apparatus according to claim 1 in which said transistor is connected as a grounded base amplifier in said oscillation circuit. 

1. A piezoelectric transformer comprising a piezoelectric member having a pair of driving electrodes at one end and an output electrode at the opposite end of said piezoelectric member, a transistor having a base electrode, a collector electrode and an emitter electrode, a transformer having a primary winding and a secondary winding, means to connect said collector electrode to one pole of a DC source through said primary winding and means to connect in series said secondary winding and said driving electrodes across the base and emitter electrodes of said transistor, thereby forming an oscillatioN circuit for producing a high voltage driving voltage, and including a phase adjuster having a variable resistor connected between one of the driving electrodes and said base electrode and a capacitor connected between said one driving electrode and said emitter electrode.
 2. The piezoelectric transformer according to claim 1 wherein the other pole of said DC source is grounded through a capacitor.
 3. The piezoelectric transformer according to claim 2 wherein a shunt resistor is connected in parallel with said capacitor.
 4. Apparatus according to claim 1 and including a diode connected between said collector and emitter electrodes and poled to protect said collector electrode.
 5. Apparatus according to claim 4 and including a diode connected between said base and emitter electrodes and poled to protect said emitter electrode.
 6. Apparatus according to claim 1 and including a resistor voltage divider connected across said DC source with an intermediate point on said voltage divider connected to said base electrode for biasing the collector base and emitter base junctions.
 7. Apparatus according to claim 1 in which said transistor is connected as a grounded base amplifier in said oscillation circuit. 